1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus using the same. The present invention is suitable for an image forming apparatus such as a laser beam printer, a digital copying machine, or a multi-function printer, which adopts an electrophotography process.
2. Description of the Related Art
Various color image forming apparatuses have conventionally been proposed, each of which includes a light source unit, multiple light scanning optical systems having imaging optical systems, and a deflection unit common among the multiple light scanning optical systems, and guides light fluxes emitted from the multiple light scanning optical systems onto multiple corresponding image bearing members to form a color image (see Japanese Patent Application Laid-Open No. 2004-021133 and Japanese Patent Application Laid-Open No. 2004-317790).
As an fθ lens used for such an optical scanning apparatus, a plastic lens has been applied for the purpose of easily producing a lens surface of an aspherical shape, which achieves highly accurate optical performance. Various methods have conventionally been proposed for accurately fixing a plastic lens to a casing (housing member) (see Japanese Patent Application Laid-Open No. H07-199100).
FIG. 13 is a main portion perspective view illustrating a method of fixing the plastic lens to the casing (housing member) proposed in Japanese Patent Application Laid-Open No. H07-199100.
In FIG. 13, a plastic lens 300 includes a lens portion 301 and a short side rib 302 and a long side rib 303 formed to surround the lens portion 301. On a side of the long side rib 303 to be abutted on a standard positioning surface 601 of a casing (housing member) 600, a protrusion 304 is disposed at a center position of a lens scanning direction (arrow X of FIG. 13). The protrusion 304 is fitted and engaged with a recessed part 602 of the casing (housing member) 600 to determine a position in the scanning direction. Thus, thermal expansion of the plastic lens 300 can be released to both ends by setting a center of the scanning direction X as a standard.
However, the optical scanning apparatus described above has the following problems.
During driving of a scanner motor to rotate a rotational polygon mirror, in the optical scanning apparatus, heat generated from an IC chip disposed on a substrate of the scanner motor or from the scanner motor causes a temperature increase and the rotation of the rotational polygon mirror generates an air flow, leading to a nonunfiorm temperature distribution.
Especially, the imaging lens disposed in proximity of the scanner motor is easily subject to the heat generated from the IC chip (motor driving circuit) disposed on the substrate of the scanner motor or from the scanner motor, and also to the air flow generated by the rotation of the rotational polygon mirror.
Air generated on a surface of the imaging lens flows from an image writing start side of the imaging lens surface to an image writing complete side thereof. Thus, in the imaging lens closest to the scanner motor, such a phenomenon occurs in which a temperature of the image writing start side of a scanning direction is higher than that of the image writing complete side.
FIG. 14 is a graph illustrating a result of measuring a surface temperature of the imaging lens when the scanner motor is continuously driven. As illustrated in the graph, a temperature difference of about 2° C. to 5° C. is generated between the image writing start side and the image writing complete side. The following problems occur when the temperature of the imaging lens asymmetrically increases in the scanning direction.
When a temperature of the plastic lens increases, a refractive index of a normal material decreases. The decreased refractive index reduces refractive power (power) of the imaging lens, and hence a beam reaching position on a scanned surface moves in a direction away from an optical axis, thereby increasing a so-called imaging magnification (printed width).
In addition to a change in refractive index, because of use of a resin material having a large linear expansion coefficient, the temperature increase is accompanied by expansion of a lens shape. When the plastic lens 300 expands with a center of the scanning direction X set as a standard as in the case of Japanese Patent Application Laid-Open No. H07-199100, a printed position moves in a direction away from an optical axis, increasing the imaging magnification (printed width).
As a result, the following problems occur especially in a color image forming apparatus illustrated in FIG. 15.
FIG. 15 is a main portion sectional view when a part of a conventional scanning unit is viewed from a rotational axis direction A of an optical deflector (polygon mirror).
As illustrated in FIG. 15, in a system that includes one shared optical deflector 28 and imaging lenses 29 arranged at the left and right of the optical deflector 28 and scans multiple scanned surfaces, an image writing start side and an image writing complete side are reversed between left and right imaging optical systems LB. In this configuration, a light emitted from a light source unit 25 passes through a collimator lens 26 and a cylindrical lens 27 to reach the optical deflector. As described above, the temperature increase of the imaging lens in proximity of the optical deflector is large at the image writing start side and small at the image writing complete side. Thus, a refractive index fluctuation or a beam reaching position fluctuation caused by thermal expansion is large at the image writing start side and small at the image writing complete side. As a result, the image writing start side and the image writing complete side are reversed at image ends (ends of the scanned surface) between the left and right imaging optical systems LB at the optical deflector 28, and hence a size relationship of printed position displacement is reversed at the images ends (ends of the scanned surface).
Thus, in the color image forming apparatus that superimposes multiple colors, imaged position displacement and registration displacement become conspicuous, and hence image quality is deteriorated.
Especially in recent years, a higher speed of the scanner motor that drives the optical deflector has been accompanied by a larger amount of heat generated from the IC chip or the scanner motor, and with miniaturization of the light scanning optical system, the imaging lens has been disposed in proximity of the heat-generating scanner motor to be more easily affected by heat.